THE SCENTED CALLA 



forms of life through the cumulative effect of 

 slight variations. 



The mutation theory is thus in many ways 

 acceptable. But to give the theory finality it is 

 obviously necessary to proceed one step farther 

 and ask this question: What causes mutation? 

 And it is equally obvious that the question must 

 be hard to answer. 



Professor de Vries, to be sure, made the 

 assumption that the changes in his evening prim- 

 rose were probably due to altered conditions of 

 nutrition incident to the growth of the plant in a 

 new soil. He further developed a thesis that 

 probably all species are subject to mutation 

 periods, which recur at more or less regular 

 periods of their life history, and which thus 

 ensure a degree of variation that will make racial 

 evolution possible. 



The authority of de Vries sufficed to give wide 

 vogue to his theory; yet it must be admitted that 

 the explanation offered lacks tangibility and at 

 best amounts to little more than begging the 

 question. 



To say that altered nutrition produces varia- 

 tion in a plant is in effect to state the fundamental 

 truth that all plants are more or less responsive 

 to their environment. 



But there is nothing specific in the case 



[95] 



